We have taken advantage of the improved time resolution (< 50 ns) of a newly developed laser stroboscopic system to study the early stages of vapor bubble nucleation in a thermal ink jet printhead. A transparent channel printhead provides optical access to vapor bubble formation and drop generation processes. Vapor bubble nucleation was found to proceed from the initial formation of localized microbubbles (< 50 ns) that spread with time until a uniform sheet of vapor covers the entire heater surface. The time duration for this process was 400 to 600 ns, depending on the heater voltage. The critical role of the heater surface is demonstrated clearly by the fact that preferential vapor microbubble nucleation sites are observed to persist over long time periods. The region of the heater surface subjected to vapor bubble collapse exhibits different microbubble properties than the rest of the heater surface.